1. Field of the Invention
The present invention relates generally to an apparatus for measuring a distance between a reference location and an article to be measured using a light wave by irradiating, e.g., a laser light beam to the article and receiving the light beam reflected from the article so as to determine a time that elapses from the irradiation of the light beam until the receipt of the reflected light beam, wherein the distance is determined based on the foregoing time that elapses during the round trip of the irradiated light beam. More particularly, the present invention relates to an apparatus for measuring a distance between a first vehicle and a second vehicle running ahead of the first vehicle using a light wave in order to prevent a collision of the first vehicle with the second vehicle based on the measured distance therebetween.
2. Background Art
To facilitate understanding of the present invention, a typical conventional apparatus 90 of the aforementioned type as shown in FIG. 6 will be described below with reference to FIG. 6 to FIG. 9.
The apparatus 90 includes an oscillator 91, a driving circuit 92 and a light generating section 93. In response to an output from the oscillator 91 via the driving circuit 92, measurement light beam L1 is irradiated from the light generating section 93 toward an article X to be measured. Subsequently, reflected light beam L2 is received by a light receiving section 94 which in turn generates a measurement signal S1. The measurement signal S1 is amplified in an amplifying circuit 95 in which its shape is properly corrected. Thereafter, the measurement signal S1 is input into a digital mix-down circuit 96 in which a phase difference between the measurement signal S1 and a reference signal S2 output from a local oscillator 97 is determined. Subsequently, a processing unit 98 including a microcomputer and other elements is activated to calculate a distance between the apparatus 90 and the article X based on the result derived from the determination of the phase difference and then display it on the screen of a displaying unit 99.
FIG. 7 is an illustrative view which shows the detailed structure of the digital mix-down circuit 96. As is apparent from the drawing, the digital mix-down circuit 96 is constructed of two-stage flip-flop circuits comprising a flip-flop circuit U1 on the input side and a flip-flop circuit U2 on the output side. Specifically, the reference signal S2 is input into a data terminal D of the FF circuit U1, and at the same time, the measurement signal S1 is input into a clock terminal Ck of the FF circuit U1. An output from an output terminal Q1 of the FF circuit U1 is input into a clock terminal Ck of the FF circuit U2, and at the same time, a data terminal D is electrically connected to an inverted output terminal Q2 of the FF circuit U2. The FF circuit U2 is electrically connected to the processing unit 98 via an output terminal Q1.
FIG. 8 shows wave shape charts, each of which illustrates a mode of operation of the digital mix-down circuit 96. First, the reference signal S2 which has been input into the data terminal D of the FF circuit U1 is latched by the measurement signal S1 which has been input into the clock terminal Ck of the same, and thereafter, the foregoing latched state is output to the output terminal Q1 of the FF circuit U2 as a "H" level.
When it is assumed that a frequency of the reference signal S2 is the same as that of the measurement signal S1 at this time, this means that the measurement signal S1 is delayed from the reference signal S2 by a time corresponding to the round trip time of the irradiated light beam between the apparatus 90 and the article X. Thus, as measurement is repeatedly conducted, the delay time is sequentially added to the preceding delay time until the measurement signal S1 is delayed from the reference signal S2 by a time corresponding to a half frequency. Consequently, the reference circuit S2 is latched by the measurement signal S1 at a "L" level, causing the reference signal S2 to be output to the output terminal Q1 of the FF circuit U2 at the "L" level. Subsequently, the result of measurement is repeatedly output from the output terminal Q1 of the FF circuit U2 at the "L" level or at the "H" level, depending on a frequency difference between the measurement signal S1 and the reference signal S2. In other words, the digital mix-down circuit 96 serves as a so-called heterodyne circuit the output of which is attenuated in the FF circuit U2 to exhibit a half frequency which in turn is input into the processing unit 98 to calculate the distance between the apparatus 90 and the article X.
With the conventional digital mix-down circuit 96 as described above with reference to FIG. 6 to FIG. 8, because of the fact that the round trip time of the irradiated light beam between the apparatus 90 and the article X is basically very short, there arises a malfunction that the measurement signal S1 erroneously latches a reverse level to a level to be originally latched due to slight fluctuation of circuit conditions or measurement conditions within a period of several frequencies before and after the time when the reference signal S2 shifts from the "L" level to the "H" level, causing an error to be induced. Similarly, this error is induced also within a period of several frequencies before and after the time when the reference signal S2 shifts from the "H" level to the "L" level. Consequently, a problem to be solved is that measurement accuracy is remarkably reduced.